Experimental evidence of grain size-shape correlation effects on the critical-state shear strength of granular media

The mechanical behavior of granular media is strongly influenced by particle geometry, including both size and shape. While critical-state shear strength is widely reported to be insensitive to particle size distribution (PSD) in assemblies of grains sharing a common shape, grain shape itself has a strong impact on strength, with angular particles generally producing higher resistance than rounded ones. Many natural and industrial granular materials, however, combine broad size distributions with significant variability in particle shape, often correlated with particle size. Whether the classical PSD-insensitivity of shear strength still holds when particle shape varies across size classes remains largely unexplored. Here, we present a systematic experimental investigation of sheared granular assemblies composed of injection-molded particles with controlled sizes and shapes. Mono-, bi-, and poly-shaped mixtures built from six characteristic grain geometries were tested under triaxial compression. We show that the classical PSD-insensitivity of critical-state shear strength breaks down when particle shape becomes size-dependent. For a given PSD, assemblies with angular small grains and rounded large grains exhibit significantly higher shear strength than those with the opposite size–shape arrangement. This behavior persists over a wide range of PSDs and can be explained by enhanced geometrical interlocking promoted by angular small particles filling the voids between larger grains.